Multi audio tones to sound card using portaudio

Question

I am trying to generate a tone to the sound card (Frequency: 1950 hz, duration: 40 ms, level: -30 db, right-channel (stereo), on steam 1). Eventually, I would like to play two of these tones (one goes to channel 1 and one goes to channel 2).

Any help or direction is greatly appreciated.

Thanks, DW

---

Hi Bjorn, I tried this but I am not getting the what I am expecting as a frequency (plus seems sound is not clean). Any ideas what's wrong? I greatly appreciate any help.

#define SAMPLE_RATE   (44100)
#define TABLE_SIZE   (200)
float FREQUENCY = 422;

...
for(int i=0; i<TABLE_SIZE; i++ )
{
    data.sine[i] = (float) sin( (double)i * ((2.0 * M_PI)/(double)SAMPLE_RATE) * FREQUENCY );
}

data.left_phase = 0;
data.right_phase = 0;
...

... in callback function ...
for(unsigned long i = 0; i < framesPerBuffer; i++ )
{   

  // fill output buffer with sin wave
  *out++ = data->amp * data->sine[data->left_phase];  // left     
  *out++ = data->amp * data->sine[data->right_phase]; // right                  

  data->left_phase += 1;    

  if( data->left_phase >= TABLE_SIZE ) 
    data->left_phase -= TABLE_SIZE;           

  data->right_phase += 1; 

  if( data->right_phase >= TABLE_SIZE ) 
    data->right_phase -= TABLE_SIZE;
} 

Answer 1

PortAudio has sample code for generating tones, you just need to figure out the frequency. See for example this answer:

[portaudio]Transmit and Detect frequency - Windows

Update:

Rather than trying to store a table of sine data, simply calculate the sine value in the callback using this formula:

amplitude[n] = sin( n * desiredFreq * 2 * pi / samplerate )

so (untested) your code will look something like this:

typedef struct
{
    long n;
} MyData;

float FREQUENCY = 422;

static int MyCallback(  
                    const void *inputBuffer, 
                    void *outputBuffer,
                    unsigned long framesPerBuffer,
                    const PaStreamCallbackTimeInfo* timeInfo,
                    PaStreamCallbackFlags statusFlags,
                    void *userData 
                  )
{   
    MyData *data = (MyData*)userData;
    float *out = (float*)outputBuffer;    

    (void) timeInfo; /* Prevent unused variable warnings. */
    (void) statusFlags;
    (void) inputBuffer;     

    for(unsigned long i = 0; i < framesPerBuffer; i++ )
    {               
        // fill output buffer with sin wave
        float v = sin( data->n * FREQUENCY * 2 * PI / (float) SAMPLERATE )
        *out++ = v; // left         
        *out++ = v; // right
    }   

    return paContinue;
}

This code is not without problems: eg. eventually n will "wrap around" and I'm not sure if sin remains accurate and efficient as the input gets larger. Nevertheless it's a good starting point, and if you just need to generate a few seconds of a tone on modern hardware, this is really all you need. If you need something fancier, get this working first, then you can worry about making it more efficient and robust with a LUT.